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Date: Thu, 29 May 2008 23:52:34 +0200
From: Fabrice Bellard <fabrice@bellard.org>
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Subject: Re: [Qemu-devel] Re: KQEMU code organization
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To: qemu-devel@nongnu.org

Anthony Liguori wrote:
> [...]
> FWIW, the l1_phys_map table is a current hurdle in getting performance. 
> When we use proper accessors to access the virtio_ring, we end up taking
> a significant performance hit (around 20% on iperf).  I have some simple
> patches that implement a page_desc cache that cache the RAM regions in a
> linear array.  That helps get most of it back.
> 
> I'd really like to remove the l1_phys_map entirely and replace it with a
> sorted list of regions.  I think this would have an overall performance
> improvement since its much more cache friendly.  One thing keeping this
> from happening is the fact that the data structure is passed up to the
> kernel for kqemu.  Eliminating that dependency would be a very good thing!

If the l1_phys_map is a performance bottleneck it means that the
internals of QEMU are not properly used. In QEMU/kqemu, it is not
accessed to do I/Os : a cache is used thru tlb_table[]. I don't see why
KVM cannot use a similar system.

Fabrice.